1. Field of the Invention
The present invention relates to a driver circuit for an active matrix display and, more particularly, to reductions in electric power consumed by the active matrix display.
2. Description of the Related Art
An active matrix display has pixels disposed at intersections. Each pixel is provided with a switching device. Information about an image is controlled by turning on and off each switching device. A liquid crystal material is used as a display medium in such a display device. In the present invention, a thin-film transistor (TFT) having three terminals (i.e., gate, source, and drain) is used as each switching device.
In the present specification, rows of a matrix construction signify signal lines (gate lines) extending parallel to the rows and connected with the gate electrodes of transistors in the rows. Columns means signal lines (source lines) extending parallel to the columns and connected with source (or drain) electrodes of transistors in the columns. A circuit for activating the gate lines is referred to herein as a gate driver circuit. Also, a circuit for activating the source lines is referred to herein as a source driver circuit. Furthermore, thin-film transistors are often referred to herein as TFTs.
In the gate driver circuit, the same number of shift registers as gate lines arranged in the vertical direction are connected in a line and in series, to produce vertical scanning timing signals for an active matrix display. In this way, the gate driver circuit turns on and off each TFT inside the active matrix display.
In the source driver circuit, the same number of shift registers as source lines arranged in the horizontal direction are connected in a line and in series, to provide a display of the horizontal components of image data to be displayed on the active matrix display. The analog switches are turned on and off by latch pulses synchronized with the horizontal scanning signal. In this manner, the source driver circuit selectively activates the TFTs inside the active matrix display and controls the orientation of each pixel cell. Signals applied to the prior art active matrix display are shown in FIG. 3. These signals applied to the active matrix display assume analog form. One frame of image is composed of two fields. A phase conversion is made every field.
In FIG. 3, the voltage Vs of the image signal and a voltage V1 applied to the common electrode are shown. Since the voltage Vs is applied to the electrode at each pixel, a differential voltage Vs-V1 is applied across the pixel cell positioned between the electrode and the common electrode. The phase of the voltage Vs is inverted every field, and as a result, the voltage applied to each pixel cell is a substantially symmetrical AC voltage. In this way, the DC voltage remaining on each pixel cell is reduced. This prolongs its lifetime.
The electric power consumed by the active matrix display can be reduced effectively by lowering the frequency at which the applied voltage is inverted.
However, as the period of the inversion of the phase of the voltage applied to the active matrix display is increased, an electric charge is drawn into each TFT when it is turned on, since the gate of the TFT has a capacitive component. As a result, a voltage difference is produced between the voltage of the analog image signal applied to the active matrix display and the voltage applied to the common electrode, the difference corresponds. The drawn electric charge, and causes a flicker. Further, each individual active matrix liquid crystal display has different characteristics. Where deterioration of the used liquid crystal material is taken into account, it is impossible to reduce the inversion frequency of the applied voltage by the same amount for every display device. Accordingly, there is a need for a simple method of adjusting the inversion frequency of the applied voltage according to the characteristics of each individual active matrix display.